This invention relates to catalysts for the polymerization of olefins, particularly for the polymerization of ethylene or the copolymerization of ethylene with other olefins. It also relates to polymerization processes employing such catalysts.
Polyolefins such as polyethylene can be produced by polymerizing olefins using catalysts comprising a transition metal compound and an organometallic compound, i.e., so-called Ziegler type catalysts. On an industrial scale, polyolefins are produced by the suspension polymerization, the solution polymerization, the gas phase polymerization or the bulk polymerization at high temperatures and high pressures using Ziegler type catalysts. The conventional Ziegler type catalyst, e.g., a catalyst comprising titanium trichloride and diethylaluminum chloride is low in the catalyst activity and accordingly, the catalyst residue remaining in the polymer formed is large in amount and causes discoloration of the polymer formed or deterioration of the polymer formed by heat or oxidation. Thus it is required to purify the polymer by a complicated step of removing catalyst residue. The trend of the times is to develop a process of compact type saving energy which employs a highly active catalyst capable of eliminating the step of removing catalyst residue from the polymer formed. As highly active catalysts, there are disclosed, for example, catalysts comprising a titanium compound supported on a magnesium compound and an organometallic compound (see Japanese Patent Publication Nos. 13050/1968, 1060/1972, 33568/1971 and 34092/1971); catalysts comprising a solid obtained by reducing a transition metal compound with an organomagnesium complex and an organometallic compound (see U.S. Pat. Nos. 4,004,071, 3,989,878 and 4,027,089); and catalysts comprising a solid obtained by reacting a reaction product of an organomagnesium complex and a halogenating agent with an organometallic compound (see U.S. Pat. No. 4,159,965 and Japanese Patent Application (OPI) Nos. 146290/1978, 26905/1981, 28206/1981, 32504/1981 and 47408/1982). Although these catalysts are highly active and compact processes using these catalysts may be achieved, there are still problems to be improved. Also, it is possible to produce polyethylenes having a low density by copolymerizing ethylene with other olefins with Ziegler type catalysts, but in order to efficiently use olefins, catalysts having a better copolymerizability are desired. Still there remain problems peculiar to each of the polymerization methods. For example, with increased polymerization temperatures in the solution polymerization and the bulk polymerization, removal of the polymerization heat evolved becomes easy and due to the decrease in the solution viscosity or the polymer viscosity it is possible to increase the solution concentration or the polymer concentration, resulting in an increase in the amount of a product. However, when the polymerization temperature is raised, the catalyst activity is decreased and as a result, it becomes difficult to produce polymers having a low melt index. On the other hand, in the suspension polymerization and the gas phase polymerization, development of catalysts capable of giving polymers having a good particle size distribution and a high bulk density is desired because shipping of polyolefins in the form of powder is possible and accordingly, a step of pelletizing the polymers obtained can be eliminated if the polymers having a good particle size distribution and a high bulk density since the polyolefins are generally shipped in the form of pellets. Furthermore, the improvement on the particle size distribution and the bulk density of polymers is an important factor for a continuous and stable operation of the suspension polymerization and the gas phase polymerization for a long period of time. Thus, there are demanded catalysts having high performances in all the aspects such as a good controllability of molecular weight distribution, a good copolymerizability, a high catalyst activity at high temperatures of polymerization, a good controllability of melt index and an excellent particle size distribution of polymers in the suspension polymerization and the gas phase polymerization as well as a high catalyst activity.